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-rw-r--r--Documentation/networking/timestamping.txt22
1 files changed, 13 insertions, 9 deletions
diff --git a/Documentation/networking/timestamping.txt b/Documentation/networking/timestamping.txt
index e8c8f4f06c67..98097d8cb910 100644
--- a/Documentation/networking/timestamping.txt
+++ b/Documentation/networking/timestamping.txt
@@ -172,15 +172,19 @@ struct skb_shared_hwtstamps {
};
Time stamps for outgoing packets are to be generated as follows:
-- In hard_start_xmit(), check if skb_tx(skb)->hardware is set no-zero.
- If yes, then the driver is expected to do hardware time stamping.
+- In hard_start_xmit(), check if (skb_shinfo(skb)->tx_flags & SKBTX_HW_TSTAMP)
+ is set no-zero. If yes, then the driver is expected to do hardware time
+ stamping.
- If this is possible for the skb and requested, then declare
- that the driver is doing the time stamping by setting the field
- skb_tx(skb)->in_progress non-zero. You might want to keep a pointer
- to the associated skb for the next step and not free the skb. A driver
- not supporting hardware time stamping doesn't do that. A driver must
- never touch sk_buff::tstamp! It is used to store software generated
- time stamps by the network subsystem.
+ that the driver is doing the time stamping by setting the flag
+ SKBTX_IN_PROGRESS in skb_shinfo(skb)->tx_flags , e.g. with
+
+ skb_shinfo(skb)->tx_flags |= SKBTX_IN_PROGRESS;
+
+ You might want to keep a pointer to the associated skb for the next step
+ and not free the skb. A driver not supporting hardware time stamping doesn't
+ do that. A driver must never touch sk_buff::tstamp! It is used to store
+ software generated time stamps by the network subsystem.
- As soon as the driver has sent the packet and/or obtained a
hardware time stamp for it, it passes the time stamp back by
calling skb_hwtstamp_tx() with the original skb, the raw
@@ -191,6 +195,6 @@ Time stamps for outgoing packets are to be generated as follows:
this would occur at a later time in the processing pipeline than other
software time stamping and therefore could lead to unexpected deltas
between time stamps.
-- If the driver did not call set skb_tx(skb)->in_progress, then
+- If the driver did not set the SKBTX_IN_PROGRESS flag (see above), then
dev_hard_start_xmit() checks whether software time stamping
is wanted as fallback and potentially generates the time stamp.